In the temperature domain below T c , the C and CC propellers are in equilibrium
but the transient conformers of intermediate geometries (at higher vibrational levels)
are less populated and hence less influential on the overall chiroptical responses.
Accordingly, the intensity of CD is straightforwardly determined by the population
of relaxed C and CC propellers at given temperature, and the experimental CD
spectrum is well reproduced in intensity (and in pattern) by the theoretical calculations assuming the exclusive population to the C conformation at the lowest temperature [8]. The relative preference for the more stable C over CC conformer
explains the CD intensity of H6 increased by lowering temperature (in this temperature domain). Along with the increase of Cotton effects, the excitation energy of H6
is gradually blue-shifted with decreasing contribution of the CC conformer at lower
temperatures. In contrast, H1 shows the opposite temperature-dependence behavior
in this temperature domain. This is attributable, at least in part, to the different tilt
angles between the relaxed C and CC conformers. Thus, more stable C conformer is
supposed to bear closer angles to the perpendicular tilt and accordingly leads to
weaker CD intensity for this conformer of H1.
In the temperature domain above T c , the CD intensity becomes more sensitive to
temperature and the Cotton effects gradually fade out by increasing temperature for
both H1 and H6. The contribution of the whizzing toroids, which are maximized at
the perpendicular blade angle, becomes more substantial with increasing population
to the transient conformers in upper vibrational levels at higher temperatures. The
excitonic coupling theory predicts that the chiroptical responses essentially vanish
when two chromophores (more precisely, two transition moments) are perpendicular
to each other [63–65], which is exactly the case with our observations. We consider
that the whizzing toroids is not a discrete static conformation with six orthogonal
radial aromatic rings but is better described as an ensemble of the intermediate
geometries populated in between the C and CC conformers on the potential surface
along the twist angle, contribution of which becomes larger at higher temperatures.
Polar solvents drive the blade geometry analogous to the whizzing toroids by an
effective solvation on the polar group(s) at the periphery. It has been already reported
that the radial aromatic rings in HABs are perpendicular to the central ring at least in
the NMR timescale in chloroform at an ambient temperature [66].
The toroidal interaction has been extensively investigated recently using a variety
of HABs as model systems [34, 43]. The localization and delocalization of exciton,
charge, and electron have been quantitatively discussed for this unique intramolecular π-interactions [44, 45, 48, 52]. In all the reported cases, the radial aromatic rings
are assumed to align in perpendicular orientation and the rotation of rings is not
considered. In our chiral HABs, the apparent blue-shifts observed for the UV and
fluorescence excitation spectra of HABs in polar solvents and at higher temperatures
are indicative of the H-type association of radial aromatic rings, where the propeller
blades are more or less aligned face to face. In contrast, the partially π-overlapped
J-type association becomes favored at lower temperatures in non-polar solvents, as
evidenced by the bathochromic shift in UV and fluorescent excitation spectra
[62, 67, 68]. The fluorescence lifetime examinations of H6 also support the existence
of the second conformer. All of the above observations are well comprehended by
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
163
but the transient conformers of intermediate geometries (at higher vibrational levels)
are less populated and hence less influential on the overall chiroptical responses.
Accordingly, the intensity of CD is straightforwardly determined by the population
of relaxed C and CC propellers at given temperature, and the experimental CD
spectrum is well reproduced in intensity (and in pattern) by the theoretical calculations assuming the exclusive population to the C conformation at the lowest temperature [8]. The relative preference for the more stable C over CC conformer
explains the CD intensity of H6 increased by lowering temperature (in this temperature domain). Along with the increase of Cotton effects, the excitation energy of H6
is gradually blue-shifted with decreasing contribution of the CC conformer at lower
temperatures. In contrast, H1 shows the opposite temperature-dependence behavior
in this temperature domain. This is attributable, at least in part, to the different tilt
angles between the relaxed C and CC conformers. Thus, more stable C conformer is
supposed to bear closer angles to the perpendicular tilt and accordingly leads to
weaker CD intensity for this conformer of H1.
In the temperature domain above T c , the CD intensity becomes more sensitive to
temperature and the Cotton effects gradually fade out by increasing temperature for
both H1 and H6. The contribution of the whizzing toroids, which are maximized at
the perpendicular blade angle, becomes more substantial with increasing population
to the transient conformers in upper vibrational levels at higher temperatures. The
excitonic coupling theory predicts that the chiroptical responses essentially vanish
when two chromophores (more precisely, two transition moments) are perpendicular
to each other [63–65], which is exactly the case with our observations. We consider
that the whizzing toroids is not a discrete static conformation with six orthogonal
radial aromatic rings but is better described as an ensemble of the intermediate
geometries populated in between the C and CC conformers on the potential surface
along the twist angle, contribution of which becomes larger at higher temperatures.
Polar solvents drive the blade geometry analogous to the whizzing toroids by an
effective solvation on the polar group(s) at the periphery. It has been already reported
that the radial aromatic rings in HABs are perpendicular to the central ring at least in
the NMR timescale in chloroform at an ambient temperature [66].
The toroidal interaction has been extensively investigated recently using a variety
of HABs as model systems [34, 43]. The localization and delocalization of exciton,
charge, and electron have been quantitatively discussed for this unique intramolecular π-interactions [44, 45, 48, 52]. In all the reported cases, the radial aromatic rings
are assumed to align in perpendicular orientation and the rotation of rings is not
considered. In our chiral HABs, the apparent blue-shifts observed for the UV and
fluorescence excitation spectra of HABs in polar solvents and at higher temperatures
are indicative of the H-type association of radial aromatic rings, where the propeller
blades are more or less aligned face to face. In contrast, the partially π-overlapped
J-type association becomes favored at lower temperatures in non-polar solvents, as
evidenced by the bathochromic shift in UV and fluorescent excitation spectra
[62, 67, 68]. The fluorescence lifetime examinations of H6 also support the existence
of the second conformer. All of the above observations are well comprehended by
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
163